Metathesis Reactions of a NHC‐Stabilized Phosphaborene

Abstract The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P‐protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N‐heterocyclic carbene (NHC)‐stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P‐terminal, as a room‐temperature‐stable crystalline solid accessible via facile NHC‐induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P‐centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X‐ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation.


Crystallographic Details
All non H-atoms were located on the electron density maps and refined anisotropically. C-bound H atoms were placed in positions of optimized geometry and treated as riding atoms. Their isotropic displacement parameters were coupled to the corresponding carrier atoms by a factor of 1.2 (CH, CH2) or 1.5 (CH3). Twinning and Disorder: The compound 2 was refined as a two-component twin (twinmatrix: -1.000 0.000 0.000, 0.000 -1.000 0.000, -0.107 0.018 1.000) using the hklf5 routine, resulting in a BASF value of 0.209(2). Disorder over two positions was observed for solvent molecules in 4 (benzene), 5 (benzene) and 6 (diethylether) and for the PSiCl2Ph unit of compound 4.

Computational Details
Geometry optimizations were performed using the Gaussian 16.C01 software. [7] All geometry optimizations were computed using the functional BP86 [8] functional with Grimme dispersion corrections D3 [9] and the Becke-Jonson damping function [10] in combination with the def2-SVP basis set. [11] The stationary points were located with the Berny algorithm [12] using redundant internal coordinates. Analytical Hessians were computed to determine the nature of stationary points (one and zero imaginary frequencies for transition states and minima, respectively) [13] and to calculate unscaled zero-point energies (ZPEs) as well as thermal corrections and entropy effects using the standard statistical-mechanics relationships for an ideal gas.
The atomic partial charges were estimated with the natural bond orbital (NBO) [14] method using NBO 7.0. [15] The topological quantum theory of atoms in molecules (QTAIM), [16] and Laplacian of the electron density analyses were carried out with AIMAII. [17] The calculations were performed at the BP86-D3(BJ)/def2-TZVPP level of theory.
TD-DFT calculations were performed using the ORCA 4.2.1 software. [18] Single point PCM/TD-B3LYP/def2-TZVPP [8a, 19] calculations were performed to estimate the change in the UV/Vis spectrum of the compounds in the presence of toluene solvent.
The nature of the chemical bonds were investigated by means of the Energy Decomposition Analysis (EDA) method, which was developed by Morokuma [20] and by Ziegler and Rauk. [21] The bonding analysis focuses on the instantaneous interaction energy ∆Eint of a bond A-B between two fragments A and B in the particular electronic reference state and in the frozen geometry AB. This energy is divided into four main components (Eq S1).
The term ∆Eelst corresponds to the classical electrostatic interaction between the unperturbed charge for charge transfer and polarization effects. [22] In the case that the Grimme dispersion corrections [9][10] are computed the term ∆Edisp is added to equation 1. Further details on the EDA method can be found in the literature. [23] In the case of the dimers, relaxation of the fragments to their equilibrium geometries at the electronic ground state is termed ∆Eprep, because it may be considered as preparation energy for S36 chemical bonding. The addition of ∆Eprep to the intrinsic interaction energy ∆Eint gives the total energy ∆E, which is, by definition, the opposite sign of the bond dissociation energy De: The EDA-NOCV method combines the EDA with the natural orbitals for chemical valence (NOCV) to decompose the orbital interaction term ∆Eorb into pairwise contributions. The NOCVs Ψi are defined as the eigenvector of the valence operator, V, given by Equation (S3).
In the EDA-NOCV scheme the orbital interaction term, ∆ , is given by Equation (S4),